U.S. patent number 5,499,277 [Application Number 08/293,868] was granted by the patent office on 1996-03-12 for method and apparatus for enhancing reactor air-cooling system performance.
This patent grant is currently assigned to General Electric Company. Invention is credited to Anstein Hunsbedt.
United States Patent |
5,499,277 |
Hunsbedt |
March 12, 1996 |
Method and apparatus for enhancing reactor air-cooling system
performance
Abstract
An enhanced decay heat removal system for removing heat from the
inert gas-filled gap space between the reactor vessel and the
containment vessel of a liquid metal-cooled nuclear reactor.
Multiple cooling ducts in flow communication with the inert
gas-filled gap space are incorporated to provide multiple flow
paths for the inert gas to circulate to heat exchangers which
remove heat from the inert gas, thereby introducing natural
convection flows in the inert gas. The inert gas in turn absorbs
heat directly from the reactor vessel by natural convection heat
transfer.
Inventors: |
Hunsbedt; Anstein (Los Gatos,
CA) |
Assignee: |
General Electric Company (San
Jose, CA)
|
Family
ID: |
23130924 |
Appl.
No.: |
08/293,868 |
Filed: |
August 19, 1994 |
Current U.S.
Class: |
376/299;
376/290 |
Current CPC
Class: |
G21C
15/18 (20130101); Y02E 30/30 (20130101) |
Current International
Class: |
G21C
15/18 (20060101); G21C 015/18 () |
Field of
Search: |
;376/299,298,290,292,293,367,368 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
0960789 |
|
Jun 1964 |
|
GB |
|
0967901 |
|
Aug 1964 |
|
GB |
|
1210048 |
|
Oct 1970 |
|
GB |
|
1247897 |
|
Sep 1971 |
|
GB |
|
1258763 |
|
Dec 1971 |
|
GB |
|
Primary Examiner: Wasil; Daniel D.
Attorney, Agent or Firm: McGinness; James E.
Government Interests
The Government of the United States of America has rights in this
invention in accordance with Contract No. DE-AC03-89SF17445 awarded
by the Department of Energy.
Claims
I claim:
1. A liquid metal-cooled nuclear reactor comprising a containment
vessel, a reactor vessel surrounded by said containment vessel with
an inert gas-filled gap space therebetween, a nuclear fuel core
arranged inside said reactor vessel, a heat collector cylinder
surrounding said containment vessel with a space therebetween, a
silo surrounding said heat collector cylinder, an air inlet duct
and an air outlet duct in flow communication with atmospheric air
external to said reactor, a cold air downcomer gap in flow
communication with said air inlet duct and extending between said
heat collector cylinder and said silo, a hot air riser gap in flow
communication with said cold air downcomer gap and said air outlet
duct and extending between said heat collector cylinder and said
containment vessel, an inert gas inlet duct and an inert gas outlet
duct in flow communication with said inert gas-filled gap space, an
inert gas downcomer duct in flow communication with said inert gas
inlet duct and an inert gas riser duct in flow communication with
said inert gas outlet duct and with said inert gas downcomer duct,
wherein said inert gas downcomer duct and said air outlet duct
share a common wall made of heat conductive material for removing
heat from said inert gas by heat exchange with atmospheric air,
wherein said inert gas downcomer duct and said inert gas riser duct
are not annular.
2. The liquid metal-cooled nuclear reactor as defined in claim 1,
further comprising thermal insulation applied to at least a portion
of the outer surface of said inert gas riser duct.
3. The liquid metal-cooled nuclear reactor as defined in claim 1,
further comprising an electromagnetic pump and a heat exchanger
arranged inside said reactor vessel, and first, second and third
baffles arranged vertically in said insert gas-filled gap space,
said first and second baffles, in conjunction with said reactor
vessel and said containment vessel, defining a first channel for
the flow of inert gas, and said second and third baffles, in
conjunction with said reactor vessel and said containment vessel,
defining a second channel for the flow of inert gas, said first
channel being in flow communication with said inert gas outlet duct
and located radially outside said heat exchanger, and said second
channel being in flow communication with said inert gas inlet duct
and said first channel and located radially outside said
electromagnetic pump.
4. The liquid metal-cooled nuclear reactor as defined in claim 1,
further comprising a stack which surrounds said air inlet duct,
said air outlet duct and said inert gas downcomer duct.
5. The liquid metal-cooled nuclear reactor as defined in claim 4,
wherein said inert gas downcomer duct communicates with said inert
gas riser duct via a horizontal duct which penetrates said
stack.
6. The liquid metal-cooled nuclear reactor as defined in claim 1,
wherein said inert gas downcomer duct and said air inlet duct share
a common wall.
7. A system for removing heat from a liquid metal-cooled nuclear
reactor in which a reactor vessel is surrounded by a containment
vessel with a fluid-filled gap space therebetween, comprising:
a fluid outlet duct in flow communication with said fluid-filled
gap space;
a fluid inlet duct in flow communication with said fluid-filled gap
space;
a fluid riser duct in flow communication with said fluid outlet
duct;
a fluid downcomer duct in flow communication with said fluid inlet
duct and said fluid riser duct; and
air circulation flowpath means in flow communication with
atmospheric air external to said reactor, wherein said air
circulation flowpath means has a first section in heat exchange
relationship with said containment vessel and a second section in
heat exchange relationship with said fluid downcomer duct, whereby
heat is removed from fluid in said fluid downcomer duct by heat
exchange with atmospheric air in said air circulation flowpath
means, wherein said fluid gas downcomer duct and said fluid riser
duct are not annular.
8. The heat removal system as defined in claim 7, further
comprising thermal insulation applied to at least a portion of the
outer surface of said inert gas riser duct.
9. The heat removal system as defined in claim 7, further
comprising a stack which surrounds said air inlet duct, said air
outlet duct and said inert gas downcomer duct.
10. The heat removal system as defined in claim 9, wherein said
inert gas downcomer duct communicates with said inert gas riser
duct via a horizontal duct which penetrates said stack.
11. The heat removal system as defined in claim 9, wherein said
stack is made of thermally insulating material.
12. The heat removal system as defined in claim 7, wherein said
fluid is an inert gas.
13. In a liquid metal-cooled nuclear reactor comprising a
containment vessel, a reactor vessel surrounded by said containment
vessel with an inert gas-filled gap space therebetween, a nuclear
fuel core arranged inside said reactor vessel, a heat collector
cylinder surrounding said containment vessel with a space
therebetween, a silo surrounding said heat collector cylinder,
first and second air inlet ducts in flow communication with
atmospheric air external to said reactor, first and second air
outlet ducts in flow communication with atmospheric air external to
said reactor, a cold air downcomer gap in flow communication with
said first and second air inlet ducts and extending between said
heat collector cylinder and said silo, a hot air riser gap in flow
communication with said cold air downcomer gap and said first and
second air outlet ducts and extending between said heat collector
cylinder and said containment vessel, the improvement comprising
first and second inert gas circulation loops in flow communication
with said inert gas-filled gap space, said first inert gas
circulation loop being in heat exchange relationship with said
first air outlet duct and said second inert gas circulation loop
being in heat exchange relationship with said second air outlet
duct, and first through fourth baffles arranged vertically in said
insert gas-filled gap space, said first through fourth baffles, in
conjunction with said reactor vessel and said containment vessel,
defining first through fourth channels for the flow of inert gas,
said first and second channels being in flow communication with
respective ends of said first inert gas circulation loop, and said
third and fourth channels being in flow communication with
respective ends of said second inert gas circulation loop.
14. The liquid metal-cooled nuclear reactor as defined in claim 13,
further comprising first and second electromagnetic pumps arranged
inside said reactor vessel at generally diametrally opposed first
and second azimuthal positions, and first and second heat
exchangers arranged inside said reactor vessel at generally
diametrally opposed third and fourth azimuthal positions
intermediate said first and second azimuthal positions, wherein
said first and third channels are located radially outside said
first and second heat exchangers respectively, and said second and
fourth channels are located radially outside said first and second
electromagnetic pumps.
15. The liquid metal-cooled nuclear reactor as defined in claim 13,
wherein each of said first through fourth baffles extends from a
highest elevation of said fluid-filled gap space to an elevation
above a lowest elevation of said fluid-filled space so that inert
gas may flow from one of said first through fourth channels to an
adjacent one of said first through fourth channels around a bottom
of a respective one of said first through fourth baffles
therebetween.
16. The liquid metal-cooled nuclear reactor as defined in claim 13,
wherein each of said first and second inert gas circulation loops
comprises an inert gas inlet duct and an inert gas outlet duct in
flow communication with said inert gas-filled gap space, an inert
gas downcomer duct in flow communication with said inert gas inlet
duct and an inert gas riser duct in flow communication with said
inert gas outlet duct and with said inert gas downcomer duct,
wherein said inert gas downcomer duct of said first inert gas
circulation loop and said first air outlet duct share a common wall
made of heat conductive material, and said inert gas downcomer duct
of said second inert gas circulation loop and said second air
outlet duct share a common wall made of heat conductive
material.
17. The liquid metal-cooled nuclear reactor as defined in claim 16,
further comprising a first stack which surrounds said first air
inlet duct, said first air outlet duct and said inert gas downcomer
duct of said first inert gas circulation loop, and a second stack
which surrounds said second air inlet duct, said second air outlet
duct and said inert gas downcomer duct of said second inert gas
circulation loop.
18. The liquid metal-cooled nuclear reactor as defined in claim 17,
wherein said inert gas downcomer duct of said first inert gas
circulation loop communicates with said inert gas riser duct of
said first inert gas circulation loop via a horizontal duct which
penetrates said first stack.
19. The liquid metal-cooled nuclear reactor as defined in claim 16,
wherein said inert gas downcomer duct of said first inert gas
circulation loop and said first air inlet duct share a common wall.
Description
FIELD OF THE INVENTION
The present invention relates generally to liquid metal-cooled
nuclear reactors and to air cooling thereof. In particular, the
invention relates to the passive removal of reactor decay and
sensible heat from a liquid metal reactor and the transport of the
heat to a heat sink (i.e., atmospheric air) by the inherent
processes of conduction and radiation of heat and natural
convection of fluids.
BACKGROUND OF THE INVENTION
In the Advanced Liquid Metal Reactor (ALMR), a reactor core of
fissionable fuel is submerged in a hot liquid metal, such as liquid
sodium, within a reactor vessel. The liquid metal is used for
cooling the reactor core, with the heat absorbed thereby being used
to produce power in a conventional manner.
A known version of an ALMR plant has a concrete silo which is
annular or circular. The silo is preferably disposed underground
and contains concentrically therein an annular containment vessel
in which is concentrically disposed a reactor vessel having a
nuclear reactor core submerged in a liquid metal coolant such as
liquid sodium. The annular space between the reactor and
containment vessels is filled with an inert gas such as argon. The
reactor and containment vessels are supported or suspended
vertically downward from an upper frame, which in turn is supported
on the concrete silo by a plurality of conventional seismic
isolators to maintain the structural integrity of the containment
and reactor vessels during earthquakes and allow uncoupled movement
between those vessels and the surrounding silo.
Operation of the reactor is controlled by neutron-absorbing control
rods which are selectively inserted into or withdrawn from the
reactor core. During operation of the reactor, it may be necessary
to shut down the fission reaction of the fuel for the purpose of
responding to an emergency condition or performing routine
maintenance. The reactor is shut down by inserting the control rods
into the core of fissionable fuel to deprive the fuel of the needed
fission-producing neutrons. However, residual decay heat continues
to be generated from the core for a certain time. This heat must be
dissipated from the shut-down reactor.
The heat capacity of the liquid metal coolant and adjacent reactor
structure aid in dissipating the residual heat. For instance, heat
is transferred by thermal radiation from the reactor vessel to the
containment vessel. As a result, the containment vessel experiences
an increase in temperature. Heat from the containment vessel will
also radiate outwardly toward a concrete silo spaced outwardly
therefrom. These structures may not be able to withstand prolonged
high temperatures. For example, the concrete making up the walls of
the typical silo may splay and crack when subjected to high
temperatures.
To prevent excessive heating of these components, a system for heat
removal is provided. One of the heat removal systems incorporated
in the ALMR is entirely passive and operates continuously by the
inherent processes of conduction and radiation of heat and natural
convection of fluids. This safety-related system, referred to as
the reactor vessel auxiliary cooling system (RVACS), is shown
schematically in FIG. 1. Heat is transported from the reactor core
to the reactor vessel 15 by natural convection of liquid sodium.
The heat is then conducted through the reactor vessel wall. Heat
transfer from the reactor vessel outside surface to the colder
containment vessel 7 across a gap space 16 filled with an inert
gas, such as argon, is almost entirely by thermal radiation. A heat
collector cylinder 4 is disposed concentrically between the
containment vessel 7 and the silo 5 to define a hot air riser 6
between the containment vessel and the inner surface of the heat
collector cylinder, and a cold air downcomer 3 between the silo and
the outer surface of the heat collector cylinder. Heat is
transferred from the containment vessel 7 to the air in the hot air
riser 6. The inner surface of heat collector cylinder 4 receives
thermal radiation from the containment vessel, with the heat
therefrom being transferred by natural convection into the rising
air for upward flow to remove the heat via air outlets 9. Heat
transfer from the containment vessel outer surface is approximately
50% by natural convection to the naturally convecting air in the
hot air riser 6 and 50% by radiation to the heat collector cylinder
4.
Heating of the air in the riser 6 by the two surrounding hot steel
surfaces induces natural air draft in the system, with atmospheric
air entering through four air inlets 1 above ground level. The air
is ducted to the cold air downcomer 3 via the inlet plenum 2, then
to the bottom of the concrete silo 5, where it turns and enters the
hot air riser 6. The hot air is ducted to the four air outlets 9
above ground level via the outlet plenum 8. The outer surface of
heat collector cylinder 4 is covered with thermal insulation (not
shown) to reduce transfer of heat from heat collector cylinder 4
into silo 5 and into the air flowing downward in cold air downcomer
3. The greater the differential in temperature between the
relatively cold downcomer air and the relatively hot air within the
riser, the greater will be the degree of natural circulation for
driving the air cooling passively, e.g., without motor-driven
pumps.
The overall heat removal rate of the RVACS increases with
temperature and is controlled to a large degree in the air riser
gap by convective heat transfer from enclosing surfaces. Thus, if
it were possible to increase the convective heat transfer on these
surfaces or increase the exposed surface area, a larger decay heat
load would be rejected by the RVACS at any given reactor assembly
temperature.
Two methods of enhancing the RVACS performance by such means are
respectively described in U.S. Pat. No. 5,043,135 to Hunsbedt et
al., entitled "Method for Passive Cooling Liquid Metal Cooled
Nuclear Reactors and Systems Thereof", and in U.S. Pat. No.
5,339,340 to Hunsbedt, entitled "Method for Enhancing Air-Side Heat
Transfer to Achieve Improved Reactor Air-Cooling System
Performance".
U.S. Pat. No. 5,043,135 describes an air-side heat transfer surface
preparation technique that results in a higher air-side convective
heat transfer rate. It involves the creation of surface roughness
by placement of protrusions 10 (see FIG. 2) that disturb the
thermal boundary layer near the hot steel walls.
An additional enhancement method described in U.S. Pat. No.
5,339,340 utilizes the air-side enhancement method of U.S. Pat. No.
5,043,135 in combination with an additional, perforated collector
cylinder 11 (see FIG. 2) placed in the air stream. The use of a
perforated steel cylinder is unique in that the degree and shape of
the perforations can be adjusted and selected such that optimum
air-side heat transfer is achieved.
The supplementary decay heat removal system which is the subject of
the present invention can be used by itself but is more effective
when used in combination with the enhancements of U.S. Pat. Nos.
5,043,135 and 5,339,340. This approach is assumed in the following
discussion.
SUMMARY OF THE INVENTION
The present invention is an improvement which seeks to enhance the
performance of the aforementioned prior art passive air cooling
system. In the enhanced decay heat removal method described herein,
heat is removed from the annular region filled with an inert gas
between the outside surface of the reactor vessel and the inside
surface of the containment vessel. This heat removal is in addition
to the heat removed by the RVACS. The enhanced method is unique in
that multiple cooling ducts in flow communication with the inert
gas-filled gap space are added to provide multiple flow paths for
the inert gas to circulate to heat exchangers. Heat in the inert
gas is removed in these heat exchangers, thereby introducing
natural convection flows in the inert gas, which in turn absorbs
heat directly from the reactor vessel by natural convection heat
transfer and acting in unison with the conventional decay heat
removal system. The total passive convective heat transfer of the
resulting dual decay heat removal system is thereby increased since
heat is removed directly from both the reactor and containment
vessel surfaces by natural convection.
The use of the prior art enhancement methods described above along
with the enhancement heat removal means of the present invention
can result in improved temperature margins in the reactor design.
In the alternative, the reactor core size could be increased for a
particular vessel size to the extent other design constraints will
permit. This could lead to a more compact and economical reactor
design for future LMR systems. Also, the passive cooling system
concept could be adapted to future, large-size reactors for which
this type of passive heat removal system has to date been shown to
be less than satisfactory because of the relatively low heat fluxes
achievable.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation view of a conventional liquid metal-cooled
nuclear reactor, showing the reactor vessel auxiliary cooling
system.
FIG. 2 is a fragmentary azimuthal sectional view of a detailed
portion of the reactor shown in FIG. 1.
FIG. 3 is an elevation view of a liquid metal-cooled nuclear
reactor in accordance with a preferred embodiment of the present
invention.
FIG. 4 is a plan view at or near the RVACS outlet plenum showing
the reactor assembly cross section and proposed ducting attached to
the containment vessel of the reactor depicted in FIG. 3.
FIG. 5 is a diagram of one RVACS stack (shown in cross section) as
modified by the present invention to include an inert gas/RVACS air
heat exchanger.
FIG. 6 is a diagram showing the flow of inert gas around one of a
plurality of baffles placed in the inert gas-filled annular space
between the reactor vessel and the containment vessel of the
reactor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The focus of the enhancements disclosed in U.S. Pat. Nos. 5,043,135
and 5,339,340 was the annular space in the air riser gap 6 between
the outside surface of the containment vessel 7 and the inside
surface of the collector cylinder 4, as shown in detail in FIG. 2.
Heat is normally transferred to the air stream from these surfaces
by convection. In the design of the conventional ALMR, these smooth
surfaces have roughness that is characteristic of commercially
available, nuclear-grade stainless steels. In accordance with the
teaching of U.S. Pat. No. 5,043,135, the surface roughness was
increased by creating surface protrusions or boundary layer trips
10 oriented in a direction essentially perpendicular to the air
flow direction, e.g., circumferentially around the cylindrical
surface of the containment vessel 7.
The enhancement of U.S. Pat. No. 5,339,340 involves placing a
perforated collector cylinder 11, having multiple openings or holes
12 as indicated in FIG. 2, in the hot air riser 6. The holes 12 can
be of arbitrary shape, although a circular shape would be the most
economical from a manufacturing standpoint. The degree of
perforation, i.e., the total surface of the openings compared to
the total perforated collector cylinder surface area, is a variable
and can be selected to provide optimum thermal performance. The
purpose of the holes 12 is to allow a fraction of the thermal
radiative heat flow emanating from the containment vessel 7 to
reach and be absorbed by the collector cylinder 4. The remainder of
the radiative heat flux is absorbed by the perforated collector
cylinder 11. Thus, the surfaces of both the collector cylinder 4
and the perforated collector cylinder 11 receive heat by radiative
heat transfer. The fraction that each will receive can be
controlled by the degree of perforation selected for the perforated
collector cylinder 11. The degree of perforation will be based on
an optimization study to achieve maximum overall convective heat
transfer from all the heat transfer surfaces in the hot air riser
6. The convective heat transfer rate to the air (the heat sink)
depends on the temperature difference between the steel surface and
air, and the convective heat transfer coefficient, which in turn
depend on the air flow velocities in the individual flow channels
created by the perforated collector cylinder 11, namely, inner
channel 13 and outer channel 14. The overall optimization process
must consider the proper positioning of perforated collector
cylinder 11 in relation to the adjacent walls of containment vessel
7 and collector cylinder 4 to achieve the desired air flow
distribution between the inner and outer flow channels. The
relative positioning of the perforated collector cylinder 11 will
also depend on the boundary layer trip configuration if these are
included in the heat transfer system.
With the air-side RVACS enhancement features of U.S. Pat. Nos.
5,043,135 and 5,339,340 included in the present design, the heat
transfer resistance in the inert-gas gap 16 between the reactor
vessel 15 and the containment vessel 7 becomes controlling. Since
almost all heat transfer in this gap is by thermal radiation, some
improvement in the overall heat rejection capability of the RVACS
might be achieved by improving the thermal emissivities of the
vessel surfaces. However, significant further increase in the
thermal emissivities of these surfaces is not possible because they
have already been increased by applying carefully prepared oxide
layers. Thus, to further improve the passive heat removal
capability, other means must be adopted.
In accordance with the present invention, enhancement of the
passive heat removal capability in an AMLR is achieved by
introducing means for removing heat directly from the inert gas in
the gap space 16 and inducing significant natural convection flows
in the gap space. The increased flow velocities in the gap space 16
result in higher convective heat transfer between the reactor
vessel 15 and the containment vessel 7. In addition, RVACS
performance is increased in an indirect manner because more draft
head and associated RVACS air flow result, as explained
hereinafter. Thus, the overall performance of the composite or dual
RVACS in accordance with the invention is increased. The degree of
increase depends to a large extent on the investment one is willing
to make in the inert gas/RVACS air exchanger.
The design and operation of the heat removal enhancement means in
accordance with the preferred embodiment of the invention is
explained with reference to FIGS. 3-6.
In accordance with the invention, four inert gas inlet ducts 21
extend horizontally into the outlet plenum and are attached to the
wall of the containment vessel 7, as indicated in FIGS. 3 and 4.
The inlet ducts 21 are in flow communication with the inert
gas-filled gap space 16 (see FIG. 2) via four inlet openings 22
(see FIG. 4). Similarly, four inert gas outlet ducts 23 are also
attached to the containment vessel at approximately the same
elevation as the inlet ducts 21 and communicate with the inert
gas-filled gap space 16 via four outlet openings 24, each of which
is positioned at an angular location which is essentially
90.degree. from the corresponding inlet opening 22, as best seen in
FIG. 5. Each reactor assembly quadrants has one inlet opening 22
and one outlet opening 24. The design is further modified by
including four vertical inert gas riser ducts 25 positioned
adjacent to the four RVACS stacks 27 along the entire length of the
stack located within the refueling enclosure 28. Each inert gas
riser duct 25 is in flow communication at its bottom end with a
corresponding one of the four inert gas outlet ducts 23, as shown
in FIG. 3, and is covered with thermal insulation 26, as shown in
FIG. 4. In addition, each riser duct 25 extends horizontally at the
top end thereof through the wall of the associated RVACS stack 27,
into the RVACS inlet ducts 29 and joining with one long side of the
rectangular RVACS outlet ducts 30, as indicated in FIGS. 3 and
4.
Inert gas downcomer ducts 31 are formed by one long side of the
RVACS outlet duct 30-which now serves as an inert gas/RVACS air
heat exchanger 32; part of the long wall of the RVACS inlet duct 29
adjacent to and opposing the heat exchanger 32; and side walls 33
formed by extending the two short side walls of the RVACS outlet
ducts 30 until they join with the opposing wall of the RVACS inlet
duct 29, as indicated in FIG. 4. Each downcomer duct 31 extends the
entire length of the associated RVACS stack 27. The bottom end of
each downcomer duct 31 joins in flow communication with the
associated inert gas inlet duct 21, as indicated in FIG. 3. The
preferred embodiment of the inert gas/RVACS air heat exchanger 32
described herein is but one of a large number of possible designs
that could be considered and which would be acceptable from a
structural point of view. However, the disclosed preferred
embodiment is considered to be the best mode because it minimizes
the modification needed to incorporate the inert gas/RVACS air heat
exchanger of the present invention in a conventional plant.
In accordance with the invention, modifications are made to the
conventional reactor assembly design as indicated in FIGS. 5 and 6.
Four flow baffles 34 are placed in the inert gas-filled gap space
16 at 90.degree. intervals along the circumference. These baffles
extend from near the top of the reactor vessel 15 and the
containment vessel 7 to near the bottom of the cylindrical portions
of the vessels. Thus, these flow baffles define four quadrants, two
of which are denoted as downflow zones 35 and two of which are
denoted as upflow zones 36. The downflow zones 35 are located at
circumferential positions corresponding to the inlet openings 22
and the upflow zones 36 are located at circumferential positions
corresponding to the outlet openings 24. Note in FIG. 5 that the
downflow zones 35 are positioned radially outside of the two sets
of electromagnetic pumps 37 whereas the upflow zones 36 are
positioned radially outside of the intermediate heat exchangers 38.
[Pumps 37 and heat exchangers 38 are conventional components
disclosed, for example, in U.S. Pat. No. 4,882,514 to Brynsvold et
al.]The reason for this orientation is that the regions of the
reactor vessel 15 outside of the intermediate heat exchangers 38
are normally hotter than the regions outside the electromagnetic
pumps 37, which will tend to promote flow patterns of the inert gas
around the bottom of each baffle as shown in FIG. 6.
During operation of the heat removal enhancement means of the
present invention, the inert gas in the two upflow zones 36 will
rise because vessel surface temperatures are higher in these zones.
The inert gas then proceeds through the four outlet openings 24
into the four inert gas outlet ducts 23 and thereafter into the
four inert gas riser ducts 25. Each inert gas riser duct is
positioned adjacent to a corresponding one of four RVACS stacks 27,
as indicated in FIGS. 3 and 5. From there the hot inert gas is
ducted into the four inert gas downcomer ducts 31, where the inert
gas is cooled by the inert gas/RVACS air heat exchangers 32. The
cooled inert gas then flows in sequence through the four inert gas
inlet ducts 21, the two downflow zones 35 and then the four inlet
openings 22. The cooled inert gas is heated as it is directed
downward, but the upward buoyancy thus created is overcome by the
much larger positive head created in the elevated inert gas/RVACS
air heat exchangers 32. The inert gas flows laterally near the
bottom of the reactor assembly, as indicated in FIG. 6, in the open
space below the end points of the flow baffles 34 and then enters
the two upflow zones 36. The inert gas is heated further as it
flows upwards in the upflow zones and then repeats the entire inert
gas flow path again.
Operation of the dual RVACS increases the decay heat removal
capability in three different ways. First, heat is removed directly
from the reactor vessel outside surface by the circulating inert
gas and transferred to the RVACS outlet air 39 in each inert
gas/RVACS air heat exchanger 32. This contribution to the
improvement in RVACS performance is by far the largest, perhaps
being as much as 90% of the total when the heat exchanger surface
area (A) is large. Second, heat is transferred to the containment
vessel 7 by the vigorous natural convection flow created in the
inert gas-filled gap space 16, which heat is in turn transferred to
the conventional RVACS air stream. Finally, the RVACS air flow rate
and thereby its performance are increased because heat is added to
the RVACS outlet air 39 in the inert gas/RVACS air heat exchangers
32, which provides increased natural circulation head and RVACS
flow rate, thus increasing air-side heat transfer coefficients as
well as surface-to-air temperature differences.
In specific preliminary analysis cases considered for the dual
RVACS concept utilizing the enhancement methods described in U.S.
Pat. Nos. 5,043,135 and 5,339,340, using a UA product parameter [UA
is the product of the heat exchanger overall heat transfer
coefficient U and the heat exchanger surface area A] value of 3320
Btu/hr-.degree.F corresponding to utilizing one long side of the
RVACS air outlet duct 30 for the inert gas/RVACS air heat exchanger
as shown in FIGS. 3 and 5, the overall performance of the dual
RVACS increased by about 13%. The corresponding reactor core power
that would be possible without reducing the RVACS temperature
margin is about 950 MW.sub.t corresponding to an estimated net
reduction in bussbar cost of 4 mills/kWh. Further increases are
possible by simply providing more heat exchanger surface area. For
the other analysis case considered, where all of the RVACS air
outlet duct was used as inert gas/RVACS air heat exchanger area, it
was determined that by using the dual RVACS of the present
invention, the reactor core power can be increased from 840 to 985
MW.sub.t (i.e., a 17.5% increase), resulting in an estimated
bussbar cost reduction of 5 mills/kWh. Such a power increase must
be consistent with other design constraints that might exist in the
current ALMR. However, if this power increase could be implemented,
significant net reduction in the electric power generating cost
could be realized.
Thus, the basic concept of the invention is that heat is removed
directly from the reactor vessel outside surface by circulating
inert gas. The heated inert gas then circulates via multiple flow
paths through heat exchangers which remove heat from the inert gas.
The cooled inert gas then flows by natural circulation back to the
annular space between the reactor vessel and the containment
vessel. This concept has been illustrated by disclosure of the
foregoing preferred embodiment. However, it is understood that this
novel concept is subject to change following trade-off and detailed
thermal performance evaluations without departing from the spirit
and scope of the invention. Also, routine variations and
modifications of the disclosed apparatus will be readily apparent
to practitioners skilled in the art of passive air cooling systems
in ALMRs. For example, the heat exchangers could also be arranged
to reject heat directly to atmospheric air which is not a part of
the RVACS air cooling stream. All such variations and modifications
are intended to be encompassed by the claims set forth
hereinafter.
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